Myron,
     Part of your explanation of the defects sounds like a material problem 
     and part sounds like a processing problem.  Some of the items listed 
     appear contradictory. Whose material are you using/getting?
     
     You say that the affected areas "usually surround barrels or via, and 
     at times span several innerlayers between barrels and/or via". If they 
     are associated with the barrel of a hole they are probably not 
     material related. Laminate and Prepreg can't tell the difference 
     between where holes will be and where they won't be. Also, If they 
     span several layers the defects you describe could not form a 
     continuous bridge across both laminate and prepreg layers.
     
     Are the defective areas touching the barrel of the hole? Or, are they 
     merely surrounding it in the clearance areas of the ground planes? Are 
     the distortions in the inner layers all moving in the same direction? 
     Say, towards the bottom or top of the board? Does it look like the 
     holes were punched instead of drilled? If either one of these is the 
     case it sounds like you have a drilling problem possibly compounded 
     with some other issue.
     
     You'll have to give more info.
     
     Dan Howell
     Zycon


______________________________ Reply Separator _________________________________
Subject: Innerlayer defects.
Author:  [log in to unmask] at corp
Date:    9/5/96 5:48 AM


     
     We are looking for an explanation as to the cause of defects found in 
     some areas of a PWB laminate.  The affected areas were observed in 
     microsections of an assembly to have the following characteristics.
     1. A dramatic difference in the color and pattern compared to the rest 
        of the innerlayer material.  Defective sections are of a very light 
        color, and are almost completely translucent.  Only small traces of 
        the usual weave pattern are seen there. 
     2. The affected areas usually surround barrels or vias, and at times   
        span several innerlayers between barrels and/or vias.
     3. Although the sides of the barrels are somewhat distorted, no cracks 
        have been seen in the plating.
     4. Distortions in the geometry of innerlayer traces are seen around    
        the affected areas.
     5. After one of the microsections was prepared, a probe was touched to 
        one of these areas, and the laminate felt soft.  In time it became  
        hardened.
     
     The assembly was microsectioned because of intermittent resistive 
     paths between certain nodes and either ground or Vcc.  At times, 
     following chamber testing, the paths had alternated between the nodes 
     and ground or Vcc!
     
     Thank you for any suggestions.
     
     Myron Papiz
     MSL
     
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